Humming noise - is it my cooler?

It may be the pump, it may also be (less likely) the waterblock/pump mount itself if at all loose. That would be the easiest fix (the only reason I even mention it) by simply ensuring all mounting screws are tight (don't strip them though). If it is the pump, it may not be an indication of eminent failure but I would suggest monitoring your temps closely.
I'm not aware of anything else - loose fan somewhere perhaps - that might cause that.

Above is a good point.

Also examine the power supply closely. A developed hum or rattle is common in cheap power supplies (that would lead to a recommendation to replace it)

Is your case well ventilated? If so I would just get a big air cooler. it will be simpler. My water cooler makes gurgling noises sometimes ... so if you care about quiet a fan seems like the best way to go.

Probably a hyper 212+ or similar.

Thanks for the responses! The pump seems to be securely screwed in, and the sound doesn't seem to be coming from my power supply. Not sure if what I have would be considered a cheap power supply: http://www.newegg.com/Product/Product.aspx?Item=N82E168... .

My case is an NXZT Phantom, which I believe has sufficient options for proper ventilation: http://www.nzxt.com/new/products/crafted_series/phantom . If it is the pump, I assume that would mean that its ability to cool is going to become compromised with time if it hasn't already. Regardless, living with this noise in my bedroom is just not an option.

I've never installed anything more complex than a graphics card before, but this video makes installing a Hyper 212 seem simple enough: http://www.youtube.com/watch?v=LAXqEftJ0Q4 . The reviews seem to say it's a very effective fan that doesn't produce a lot of noise and the price is very nice. I'll do some more research about what to expect while installing the fan, and I'll probably end up going with this if it doesn't seem to difficult.

Definitely not a cheap power supply. Good to see!

Lots of CPU cooler options to choose from, both closed loop water and air coolers. Kind of a matter of preference what you pick! silentpcreview.com has good CPU cooler reviews, and Tom's also has some good articles.

Replaced the liquid CPU with the Hyper 212+. Extremely quiet, and keeps my cpu more than cool enough.

However, while quieter than my liquid cooler, it wasn't the main culprit for all this noise. Now that I've had the time to really dig in and examine more closely, I realize it's actually been coming from UNDER the computer, where the power supply fan is. This is a bit disconcerting, considering the price of that component.

I bought the entire rig from IBUYPOWER just over a year ago, so I assumed I was covered by their warranty. However, it seems their "3 year" warranty is actually 1 year parts, 2 years labor. No point in paying shipping charges and waiting for them to get around to it when I can just repair it myself at this point. If I decide to ignore the noise (because PC money is not in the budget right now), does anyone know if power supply failure can damage the rest of system?

A faulty PSU may cause premature wear and component failure. I would be getting in touch with CoolerMaster since the PSU itself has a two year (limited) parts and labor warranty. Before doing that, you might pull the PSU out of the case (just by unscrewing the PSU at the back and shifting it out of the case - if careful you you may be able to do it without unplugging anything other than the AC into the PSU) try starting the computer with the PSU isolated from the case - just to ensure the noise is from the PSU.
Hope you find it's just a loose screw or pad or... that can easily be remedied

Thanks for the advice! I registered the PSU on the Cooler Master website, and the warranty does indeed extend until 2016. I sent customer support an email and am awaiting reply. I have never had to contact the manufacturer of a component before, so hopefully Cooler Master is not one of the those "customer support horror story" companies. As a college student heading into finals week, I hope that I can get it replaced without losing my computer for very long.

Until then, I'll have to study with my music on. This thing is getting louder each day!